This invention relates to a method and apparatus for flow measurement and more particularly, to the measurement of very slow flow rates of electrolytic body fluids.
Numerous flowmeter devices have been proposed and developed for measuring the rate of flow of fluid through various conduits. However, the flowmeters which are commonly used for measuring relatively low flow rates fail to measure the low flow rates accurately and/or simply take too long to make the measurement.
One well-known class of flowmeters is called the head meter. It functions by measuring the pressure differential across a suitable, fixed restriction to the flow to be measured such as an orifice plate, a venturi restriction, a capillary tube or other form. the pressure differential is measured with a pressure-responsive deflectable metal diaphragm. Measurements of very, however, low flow rates suffer inaccuracies due to the history of deflection and differential pressure characteristics inherent in metal diaphragms. Morever, response time may extend to minutes or hours when the flow rate is so low that it is only capable required to flex satisfying the volume displacement of the diaphragm in minutes or hours.
Another type of flowmeter is the area type having a variable orifice and a substantially constant pressure drop rather than a fixed restriction and a varying pressure drop. In the area meter, flow rate is reflected by the changing area of the opening through which the liquid must pass maintain the constant pressure. In the area flowmeter of the type disclosed by Dettmer U.S. Pat. No. 3,712,134, a vertical tube through which the fluid is conducted in the upward direction is provided with a tapered cross-sectional area. A member therein assumes a vertical position at a condition of equilibrium between the downward gravitational force on the member and the upward pressure of the fluid flow through the variable-area annular orifice which surrounds it. The position of equilibrium is therefore a function of the flow rate; the greater the flow rate, the higher the vertical position of the float. This type of variable area flowmeter, however, cannot be used for extremely low flow rates because of the friction and other forces which load the member causing large errors in drag which distort the relationship between the flow rate and the member position and thereby give rise to inaccurate readings.
Another type of flowmeter is disclosed in U.S. Pat. No. 3,450,984. It has a pair of electrodes inserted in a fluid and a current capable of electrolyzing the fluid is passed between the electrodes. Electrolysis causes gas bubbles in the current path of the electrodes. The gas bubbles are displaced at a rate depending upon the fluid flow rate, resulting in a change in the resistance between the electrodes which is an indication of the fluid flow rate. The problem with this technique is that the response time is very, very long and reproducibility is difficult to achieve because the current between the electrodes must stabilize before an accurate measurement can be made.
The above mentioned prior art flow meters and methods of measuring flow have a further disadvantage in not being capable of adequately measuring the flow of cerebrospinal fluid in a ventriculoatrial shunt and/or a ventriculoperitoneal and lumboperitoneal shunt, the need for which measurements has increased due to the increased usage of such shunts. Cerebrospinal fluid shunt flow is particularly difficult to measure due to the intermittancy and variability of the flow caused by changes in physiological conditions. Moreover, since the shunt tubing itself is generally implanted below the skin of the patient, it is highly desirable that any flow cell associated therewith also be implantable. Lastly, due to the fact that the flow measurement is in vivo and carried out during the use of the shunt on the patient, it is important that the flowmeter itself does not in any way obstruct or change the dynamics of the flow of the spinal fluid during use.